In the past, it has been customary to impregnate wood with preservatives such as pentachlorophenol by first steaming the wood, subjecting the wood to a vacuum or to air pressure, placing the so-pretreated wood in a solution of the preservative contained in a petroleum distillate solvent, soaking the wood for a period of time or forcing the solution into the wood under pressure and finally employing steam and vacuum to remove excess petroleum distillate solvent from the surface only, leaving the bulk of the oil in the wood. Such a process requires about 16 hours to complete and has proved to have several disadvantages. First, the solvents normally employed are flammable, thereby presenting a flammability hazard during the treating process. In order to avoid the flammability problem, it was necessary in the past to use a higher boiling hydrocarbon, e.g. a heavy oil. As a consequence it was more difficult to remove. The commonly employed petroleum distillate solvents likewise tend to bleed out of the wood with ageing thereby producing a toxic effect on animals and men coming in contact therewith. The phenomenon of bleeding likewise prevents paint and other coatings from properly adhering to wood. In addition, the hydrocarbon remaining in the wood oxidizes, forming colored products which darken the wood.
The prior art is replete with teachings of improved procedures for impregnating wood with various chemicals such as preservatives, fire retardants, dimensional stabilizers and the like. The more recent preservative processes have proclaimed results which, in addition to providing protection, render the wood clean, the carrier and preservative do not exude from the wood. The most widely used clean wood process is the so-called salt process, of CCA, which employs water containing soluble salts such as copper oxide, chromic oxide and arsenic acid. This process is losing its appeal due to the added expense of both clean-up of the excess treating solution to avoid environmental pollution and reseasoning (drying) after treatment. The other clean wood process in use today is known as the "Cellon" process and is based on patents issued to R. H. Bescher and assigned to Koppers Company, Incorporated, particularly Nos. 3,199,211 and 3,200,003. The (Cellon process employs a liquid petroleum gas (LPG) such as propane, butane, and isobutane, along with a high boiling cosolvent as the carrier or solubilizing agent for the preservative. Other petroleum hydrocarbons such as benzene, toluene, xylene, the heavy naphthas such as diesel oil, fuel oil, light bunker oil and the like have been suggested as carriers. The alcohols, such as methanol, have also been suggested as carriers. Hudson, in U.S. Pat. No. 2,650,885, and McDonald, in U.S. Pat. No. 2,860,070, each Disclose processes which employ chlorinated hydrocarbons such as trichloroethylene and perchloroethylene.
The technology of wood treating with preservatives can be said to begin with Rueping who discovered that reduced cost could be realized by subjecting the wood to air pressure prior to immersion in the treating solution. The air pressured into the wood is trapped by the impregnating solution and upon release of the pressure the excess solution is forced out of the wood, leaving only sufficient solution to coat the cells rather than fill them.
Such technique was the forerunner of the technique which has become known as the "empty cell" technique. The Reuping process is described in three U.S. Pats., namely U.S. Pat. No. 709,799 issued in 1902, Re. U.S. Pat. No. 12,707 issued in 1907 and U.S. Pat. No. 1,008,864 issued in 1911. Next appears a process which has become known as the "full cell" technique in which prior to impregnation the wood is subjected to a vacuum. This process is described in U.S. Pat. Nos. 2,135,029 and 2,135,030 issued to Boller and U.S. Pat. No. 2,314,362 issued to Mills.
The next innovation appears to be that of Monie S. Hudson described as a vapor drying technique. The Hudson patents, U.S. Pat. Nos. 2,435,219 and 2,650,885, employ the Rueping air pressurization, impregnation with a solvent carrier containing the treating agent followed by repeated contacting of the treated wood (after draining) with the vapors of a solvent to heat the wood together with intermittent periods of vacuum to vaporize the solvent more rapidly from the heated wood.
Following Hudson, McDonald in U.S. Pat. No. 2,860,070 disclosed a procedure wherein perchloroethylene, for example, containing the treating agent was employed following drying (seasoning in perchloroethylene, vapor-drying vacuum technique). The vapor-drying vacuum technique was thus employed specifically in the case of perchloroethylene.
Finally, Bescher in U.S. Pat. Nos. 3,199,211 and 3,200,003 disclosed the process which has come to be known as the Cellon process. The process uses the liquifiable petroleum gas (LPG) carrier and employs a purge with a non-condensable gas low in oxygen content to free the cylinder of air before and after impregnation to prevent the highly flammable vapors coming into contact with oxygen at proportions near or above their explosive limits.
Each of these prior art processes possesses one or more disadvantages. For example, as previously mentioned the Bescher Cellon process requires low oxygen content gas purge to prevent fire and explosion. Further, large quantities of the LPG are lost during the after treatment purge with gases such as nitrogen. In addition, the treated wood must be air seasoned for several weeks before use to permit the LPG remaining in the wood to equilibrate with the atmosphere. Also, upon being brought out of the cylinder, the wood is covered by needle crystals of pentachlorophenol on its surface which requires the wood to be cleaned free by brushing or alkaline washing prior to use.
The hydrocarbon treated woods, as in the Reuping and Hudson process, employing flammable solvents require careful handling to avoid the flammable or explosive limits of the particular solvent. The Hudson vapor-drying process and the Rueping process as well as the McDonald process each require vacuum systems to assist in the recovery of this solvent. In addition the Reuping process requires high pressure air for pre-impregnation and the Boller process requires a vacuum pretreatment prior to impregnation. The salt treatments are subject to different disadvantages, the primary one being the wood must be seasoned (dried) both before and after treatment. Thus, while many techniques have been employed to impregnate wood each is rife with disadvantages which have limited their acceptance, in most instances, to special situations which can afford the risk or the expense.
It therefore is an object of the present invention to provide a process employing non-flammable solvents, which solvents are substantially completely removed following impregnation, which can be substantially recovered after removal from the wood, which do not migrate to the surface and bring wood rosins and preservative to the surface, and which permit the wood to be painted immediately after removal from the cylinder. In addition, it is an object of the present invention to provide a process which does not require low oxygen or oxygen free gas purges before and/or after impregnation or air pressure or vacuum cycles before or after impregnation. It is a still further object of the present invention to provide a process which by simple recovery techniques prevents both air and water pollution at the treating site.
Another object of this invention is to provide a process whereby the solvent for the wood preservative can be substantially completely removed from the wood.
A further object of the present invention is to provide a process whereby the wood preservative is not brought to the surface of the wood by the solvent bleeding.
A still further object is to provide a process which produces a light, natural colored product and one which can be painted.
These and other objects and advantages of the present process will become apparent from the reading of the following detailed description.